After reading a good chunk of the Opus RFC, I believe it may be possible to use Opus padding to embed lossless deltas for each packet. It states that this padding may be of any size, and that while an Opus encoder itself must set any padding to zero, the decoder must accept any value. It also states that when the decoder has finished reading bytes from a frame for decoding, it may not spill over into the padding for further input. Hrm... Sounds like an arbitrary extension field to me.

My only concern is about just how much padding decoders would be willing to accept before deciding that the packet is a DOS attack.

Deltas are not going to do you any good. The opus specification is quite intentionally not bit exact, and the decoder gives slightly different output on different systems and different versions of the decoder. The design of the format also does not reliably result in compact simple lossless deltas (e.g. the phase shift in hybrid mode, or how HF gets filled with 'folded' noise).

For completeness, the RFC also specifies that the padding MUST be set to all zerosó though certainly some future revision to the RFC could make use of the freedom that comes from also mandating that the decoder ignore the padding.